Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, natural gas has been recognized as an attractive alternative fuel. For automotive applications, natural gas may be compressed and stored as a gas in cylinders at high pressure. Various engine systems may be used with CNG fuels, utilizing various engine technologies and injection technologies that are adapted to the specific physical and chemical properties of CNG fuels. For example, mono-fuel engine systems may be configured to operate only with CNG while multi-fuel systems may be configured to operate with CNG and one or more other fuels, such as gasoline or gasoline blend liquid fuels. Engine control systems may operate such multi-fuels systems in various operating modes based on engine operating conditions.
One example multi-fuel system is described by Surnilla et al. in U.S. Pat. No. 7,703,435. Therein, an engine is configured to operate on CNG, gasoline, or a mixture of both. Fuel is selected for operating the engine during particular operating conditions based on the amount of fuel available in each fuel storage tank as well as based on the type and attributes of the available fuel. For example, vehicle mileage can be extended by selecting a particular fuel during high driver demand. As another example, engine emissions can be improved by reserving a particular fuel for engine starting conditions.
However the inventors herein have recognized that the approach of '435 may not leverage all the attributes of the available fuels. For example, the approach does not take into consideration the flammability limits of the available fuels. Since the flammability limits of each fuel have an effect on the fuel's octane, torque output, and knock addressing ability, the fuel injection profiles selected may have torque loss issues, knock resistance issues, and/or may require the use of substantial spark retard. As such, any of these may result in reduced fuel economy.
In one example, some of the above issues may be addressed by an engine method that leverages all the attributes of the available fuels, including the flammability limits. The method comprises, during high load conditions, when operating with a first gaseous fuel, in response to an elevated exhaust temperature, enriching the engine via injection of a second, liquid fuel while maintaining spark timing.
As an example, an engine may be configured to operate on a first, gaseous fuel, such as CNG, and a second, liquid fuel, such as gasoline. During high load conditions, the engine may be operating on at least some CNG to provide benefits that minimize the consumption of liquid fuel while meeting the torque demand. In other words, the system may be configured to preferentially use the low cost, high octane gaseous fuel. For example, CNG may be port injected into an engine cylinder, the port injection based on the intake air received in the cylinder so as to operate the cylinder with a combustion air-fuel ratio that is substantially at or around stoichiometry. In response to elevated exhaust temperatures experienced while operating at high engine loads, exhaust cooling may be achieved by enriching the engine cylinder via increased injection of the liquid fuel. For example, as the exhaust temperature increases above a threshold temperature, gasoline may be direct injected into the cylinder to provide a combustion air-fuel ratio that is richer than stoichiometry. At the same time, injection of CNG may be maintained while also maintaining spark timing at MBT. For example, while operating the cylinder with 100% CNG (that is, fueling with CNG corresponding to 100% of the intake air amount) to meet the torque demand, the cylinder may be enriched using up to 15% gasoline (that is, fueling with additional gasoline corresponding to 15% of the intake air amount). While the example is referenced as 100% CNG and 15% gasoline, the same may also be alternatively represented as a fuel split of 87% CNG and 13% gasoline at 15% overall richness.
This approach provides various benefits. First, the octane of CNG can be used to meet the torque demand without need for spark retard while the wider flammability limit of gasoline (e.g., in the range of 0.6 to 1.5 lambda) is advantageously used to cool the exhaust. By using CNG to power the engine and gasoline to cool the engine, a smaller degree of enrichment is needed to cool the engine than would have otherwise been required if the engine were operating with only gasoline. As such, this improves fuel economy. By reducing the need for spark retard (which would have otherwise been required if the engine were operating with only gasoline), torque losses and power losses are reduced. By also reducing the need for throttle adjustments (which would have otherwise been required to reduce the exhaust temperature), torque losses due to lower intake air charge are also reduced. Overall, exhaust cooling is achieved without suffering power losses and without degrading fuel economy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.